This invention relates to the recording of a bi-level signal exhibiting high bit density and, more particularly, to a method and apparatus of recording such a bi-level signal whereby non-linear distortion is minimized.
In recording bi-level signals on a magnetic medium, such as magnetic tape, the level of the bi-level signal reproduced therefrom is reduced in a non-linear manner if the bit density suddenly is increased. Also, the phase of such a reproduced bi-level signal is subjected to non-linear distortion. Such non-linear amplitude and phase distortion is referred to as "carrier-drop" or "over-modulation" when the bi-level signal is a pulse carrier whose repetition rate, or frequency, is modulated to represent information. Such non-linear distortion is referred to as "peak-shift" when the bi-level signal is a digital signal whose binary bits represent information, such as a PCM signal, a binary coded decimal signal, or the like.
The carrier-drop, or over-modulation effect is most pronounced when the frequency of the FM pulse carrier changes abruptly from a relatively lower frequency to a higher frequency. The peak-shift effect is most pronounced when the bit content of the digital signal changes over from, for example, a string of "0"s (or a string of "1"s) to an alternating bit pattern of "0"s and "1"s. That is, when the bit density of the recorded digital signal increases, the peak-shift phenomenon is most noticeable.
In most cases, i.e. an abrupt increase in the frequency of the FM pulse carrier, or an abrupt increase in the bit density of the digital signal, non-linear amplitude and phase distortions are at a maximum at the beginning of the change-over, and thereafter gradually dissipate. Hence, if the higher FM pulse carrier frequency, or the higher bit density of the digital signal continues, the non-linear distortion in the reproduced signal gradually is reduced. Consequently, after a number of cycles of the FM pulse carrier, or after a number of bit alternations in the digital signal, the reproduced signal is recovered without such non-linear distortion.
Non-linear distortion of the aforenoted type is enhanced when the FM or digital signals are recorded with relatively high recording currents. Also, if the wavelength of the recording signal is relatively short, or if the effective distance, or gap, between the recording head and the magnetic medium is relatively large, there is a greater propensity for the signal to be reproduced with non-linear distortion.
If the FM pulse carrier represents, for example, video information, non-linear distortion therein results in a distorted reproduced video signal which can be perceived as a distorted video picture. If the recorded signal is a digital signal, then, since binary "1"s and "0"s are detected as a function of the "zero-crossing" points of the reproduced signal, the non-linear distortion, which appears as peak-shifts in the reproduced digital signal, impedes satisfactory detection of the zero-crossing points and, thus, makes it difficult to adequately detect the reproduced digital signal.
It has been thought that non-linear distortion in the reproduced FM pulse carrier can be compensated by momentarily increasing the recording current at the instant that the frequency of the FM pulse carrier changes from lower to higher frequency levels. However, this type of compensation is dependent upon a detecting circuit which functions to accurately detect this low-to-high frequency change. Such a detecting circuit is not simple to implement and, thus, adds to the complexity and cost of the recording circuitry. Furthermore, even if this frequency change-over can be detected accurately, it is difficult to maintain optimum recording current levels. If the recording current exceeds the optimum level, as may easily occur, then the resultant large recording current is, itself, a source of non-linear distortion, as mentioned above. Furthermore, in order to control the recording current levels properly, the usual recording amplifier must be an analog amplifier, as opposed to a relatively simpler, less expensive switching-type amplifier.
There has, therefore, been a need for compensating FM pulse carriers and digital signals during the recording thereof such that the signals may be reproduced with minimal non-linear distortion .